As you read in Week 1, the staining of cells and sections with antibodies has revolutionised histology as it allows the identification and localisation of individual molecules.
A molecule that binds to, and is recognised by, an antibody, is called an antigen, and, in the context of histology, such antigens are often referred to as markers, since they act as ways of recognising a particular cell. Many markers are designated by a CD number. Indeed, more than 250 markers have been assigned CD designations.
As you read previously, immunohistochemistry (IHC) is particularly valuable for distinguishing different cell types in the diagnosis of cancer. For example, different classes of lymphocyte appear virtually identical in size and shape (morphology), but they can be distinguished according to their surface markers. All T lymphocytes express CD3, and the two major subpopulations of helper T cells and cytotoxic T cells express CD4 and CD8, respectively. Therefore, a T cell lymphoma can be tracked in different tissues using these markers.
Another use for IHC is in guiding treatment. For example, many breast cancers require oestrogen to divide, but a drug called tamoxifen can bind to the oestrogen receptor on the cancerous cells, blocking this proliferative effect. However, this only applies to some cancers, as others do not have an oestrogen receptor and are therefore not susceptible to tamoxifen therapy.
A researcher or clinician can use IHC to identify whether a breast cancer removed by surgery expresses the oestrogen receptor. The clinician wants to treat the patient so as to prevent any secondary tumours from growing and with this information they can decide whether or not tamoxifen therapy is appropriate.